MRI Pulse Sequences and Image Contrast

Study Notes

Pulse sequences are a series of parameters used to create MRI images
The sequence involves RF pulses and gradients to form a MR image
Key components include: net longitudinal magnetization, transmission of RF pulses, gradient activation for localization and acquisition of signal (echo), and K-Space filling
Image acquisition involves: patient, pulse sequence, data acquisition, stored echoes in K-Space, and the resultant MR image

What is a pulse sequence?
Classification of pulse sequences: Spin Echo (SE), Modifications of SE sequences including Dual Spin-Echo, Fast (Turbo) Spin-Echo, Single-Shot Fast Spin-Echo Sequences
Gradient Echo (GRE) sequences: Main differences, types, parameters, and mechanisms
Inversion Recovery (IR) sequences; including, TI value, tissue suppression, and its uses for different tissue types
Echo Planar Imaging (EPI) sequences; filling multiple lines of K-Space, single-shot EPI (SS-EPI) and its advantages, and different types of EPI sequences including their applications in clinical MR imaging such as perfusion-weighted imaging (PWI), diffusion-weighted imaging (DWI), functional MRI (fMRI), magnetization transfer (MT) contrast, magnetic resonance angiography (MRA) and Time of Flight (TOF) MRA, Phase Contrast (PC) MRA

90° and 180° RF pulses are used, flipping magnetization vectors
The initial signal is called free induction decay (FID), weak for imaging
180° pulse rephases protons, generating a stronger signal (spin echo)
TR (Time to Repeat) is the time between 90° pulses
TE (Time to Echo) is the time between the 90° pulse and echo reception
Gradients are used for signal localization and slice selection
The sequence is used for anatomical imaging, as diseased tissues appear brighter on T2-weighted images.

Conventional SE sequences fill one line of K-Space per TR.
Dual Spin-Echo sequences use two 180° pulses per TR, providing two echoes for proton density (PD)- and T2-weighted images in different K-spaces.
Fast (Turbo) Spin-Echo sequences use multiple 180° pulses to obtain multiple echoes within one TR to increase scanning speed.
Single-Shot Fast Spin-Echo sequences acquire all echoes in one TR to dramatically reduce scan time with suitable quality. Short turbo factor decreases effective TE, increasing T1 weighting. Long turbo factor increases effective TE, increasing T2 weighting.

No 180° pulses are used
Rephasing using gradients
Smaller flip angles (~90° or less) are used to reduce scanning time, which is crucial for fast imaging.
Signal loss due to field inhomogeneity is significant in GRE
Two types of GRE sequences
- Spoiled (Incoherent) GRE sequences: Residual TM is destroyed
- Steady-state (Coherent) GRE sequences: Residual TM is refocused, resulting in a steady signal to reduce scanning time, ideal for fast dynamic or rapid physiologic processes. These also show high signal intensity in tissues with longer T2 relaxation times.

180° inversion pulse is used before the standard 90° pulse in a spin echo sequence
The inversion pulse saturates the tissues
LM recovers at different time rates for different tissues influencing contrast
Time of inversion (TI) is crucial in determining tissue contrast.
A common use is to suppress specific tissues, like fat or CSF
TI values are chosen to suppress specific tissues.

EPI sequences fill multiple lines of K-space in a single TR (repeating time), dramatically reducing scan time
If echoes are produced by multiple 180° pulses, it is referred to as spin echo echo planar imaging (SE-EPI).
If refocusing gradients are used called GE-EPI which is faster than SE-EPI
Various applications in dynamic imaging, including perfusion-weighted imaging (PWI), diffusion-weighted imaging,functional MRI (fMRI), and variations in blood vessel imaging methods including magnetization transfer contrast (MT) and magnetic resonance angiography (MRA) including Time of Flight (TOF) MRA and Phase Contrast (PC) MRA.